text: As a nuclear fuel bundle increases in burnup (time in reactor), the radiation begins changing not only the fuel pellets inside the cladding, but the cladding material itself.  The zirconium chemically reacts to the water flowing around it as coolant, forming a protective oxide on the surface of the cladding.  Typically a fifth of the cladding wall will be consumed by oxide in PWRs.  There is a smaller corrosion layer thickness in BWRs.  The chemical reaction that takes place is:
  Zr + 2 HO → ZrO + 2 H (g)
  Hydriding occurs when the product gas (hydrogen) precipitates out as hydrides within the zirconium.  This causes the cladding to become embrittled, instead of ductile.  The hydride bands form in rings within the cladding.  As the cladding experiences hoop stress from the growing amount of fission products, the hoop stress increases.  The material limitations of the cladding is one aspect that limits the amount of burnup nuclear fuel can accumulate in a reactor.
  CRUD (Chalk River Unidentified Deposits) was discovered by Chalk River Laboratories. It occurs on the exterior of the clad as burnup is accumulated.
  When a nuclear fuel assembly is prepared for onsite storage, it is dried and moved to a spent nuclear fuel shipping cask with scores of other assemblies.  Then it sits on a concrete pad for a number of years waiting for an intermediate storage facility or reprocessing.  The transportation of radiation-damaged cladding is tricky, because it is so fragile.  After being removed from the reactor and cooling down in the spent fuel pool, the hydrides within the cladding of an assembly reorient themselves so that they radially point out from the fuel, rather than circularly in the direction of the hoop stress.  This puts the fuel in a situation so that when it is moved to its final resting place, if the cask were to fall, the cladding would be so weak it could break and release the spent fuel pellets inside the cask.